Magnetoresistive Random Access Memory (MRAM), based on the integration of silicon CMOS with magnetic tunnel junction (MTJ) technology, is a major emerging technology that is highly competitive with existing semiconductor memories such as SRAM, DRAM, and Flash. Similarly, spin-transfer (spin torque) magnetization switching described by C. Slonczewski in “Current driven excitation of magnetic multilayers”, J. Magn. Magn. Mater. V 159, L1-L7 (1996), has recently stimulated considerable interest due to its potential application for spintronic devices such as STT-MRAM on a gigabit scale.
Both field-MRAM and STT-MRAM have a MTJ element based on a tunneling magneto-resistance (TMR) effect wherein a stack of layers has a configuration in which two ferromagnetic layers are separated by a thin non-magnetic dielectric layer. One of the ferromagnetic layers has a magnetic moment that is pinned in a first direction while the other ferromagnetic layer has a magnetic moment which is free to rotate in a direction that is either parallel or anti-parallel to the first direction. As the size of MRAM cells decreases, the use of external magnetic fields generated by current carrying lines to switch the magnetic moment direction of the free layer becomes problematic. One of the keys to manufacturability of ultra-high density MRAMs is to provide a robust magnetic switching margin by eliminating the half-select disturb issue. For this reason, a new type of device called a spin transfer (spin torque) device was developed. Compared with conventional MRAM, spin-transfer torque or STT-MRAM has an advantage in avoiding the half select problem and writing disturbance between adjacent cells. The spin-transfer effect arises from the spin dependent electron transport properties of ferromagnetic-spacer-ferromagnetic multilayers. When a spin-polarized current transverses a magnetic multilayer in a current perpendicular to plane (CPP) configuration, the spin angular moment of electrons incident on a ferromagnetic layer interacts with magnetic moments of the ferromagnetic layer near the interface between the ferromagnetic layer and non-magnetic spacer. Through this interaction, the electrons transfer a portion of their angular momentum to the ferromagnetic free layer. As a result, spin-polarized current can switch the magnetization direction of the ferromagnetic free layer if the current density is sufficiently high, and if the dimensions of the multilayer are small.
For STT-MRAM to be viable in the 90 nm technology node and beyond, the ultra-small MTJs (also referred to as nanomagnets) must exhibit a magnetoresistive (MR) ratio that is much higher than in a conventional MRAM-MTJ which uses a NiFe free layer and AlOx as the tunnel barrier layer. Furthermore, the critical current density (Jc) must be lower than about 106 A/cm2 to be driven by a CMOS transistor that can typically deliver 100 μA per 100 nm gate width. A critical current for spin transfer switching (Ic), which is defined as [(Ic++Ic−)/2], for the present 180 nm node sub-micron MTJ having a top-down oval shaped area of about 0.2×0.4 micron, is generally a few milliamperes. The critical current density (Jc), for example (Ic/A), is on the order of several 107 A/cm2. This high current density, which is required to induce the spin-transfer effect, could destroy a thin tunnel barrier made of AlOx, MgO, or the like. Thus, for high density devices such as STT-MRAM on a gigabit scale, it is desirable to decrease Ic (and its Jc) by approximately an order of magnitude so as to avoid an electrical breakdown of the MTJ device and to be compatible with the underlying CMOS transistor that is used to provide switching current and to select a memory cell.
Several schemes have been presented to use the spin transfer torque mechanism for magnetic based memory such as STT-MRAM, or current induced domain wall motion based MRAM, logic, and sensor applications. A preferred implementation is to employ a magnetic tunnel junction (MTJ) with a pinned ferromagnetic layer and free ferromagnetic layer separated by a tunneling oxide layer in a TMR configuration. Although this scheme has been widely studied, further improvements in overall performance are needed before a domain wall motion device is used commercially as a memory element in MRAM or as a sensor in hard disk drive (HDD) heads.
Referring to FIG. 1, the two magnetic layers in a TMR configuration can either have their magnetization pointing in the plane or out of the plane of the film. An example of in-plane magnetization is shown on side (a) of FIG. 1 where a pinned layer 10 has a magnetic moment along an x-axis and the free layer 12 has a magnetic moment free to rotate in a (+) or (−) x-axis direction. An insulating (tunnel barrier) layer 11 separates the aforementioned two ferromagnetic layers. Out of plane magnetization (PMA) is depicted on side (b) of FIG. 1 where pinned layer 20 has a magnetization pointing in a y-axis direction or perpendicular to the film plane and the free layer 21 has a magnetic moment that is free to rotate either in a (+) or (−) y-axis direction. In both examples, the free layer and pinned layer magnetizations are parallel or anti-parallel in a quiescent state. Thus, storage of the digital information as a “1” or “0” magnetic state is provided by the direction of magnetization in the free layer 12 (or 21).
In the case of a free layer having a magnetization direction perpendicular to the plane of the film, the critical current (ic) required to switch the magnetic direction in the magnetic element is directly proportional to the perpendicular anisotropy field as shown in equation (1):
                              i          c                =                              aeMsVH                                          k                eff                            ,              ⊥                                            g            ⁢                                                  ⁢            ℏ                                              (        1        )            where e is the electron charge, α is a Gilbert damping parameter, Ms is the saturation magnetization of the free layer, h is the reduced Plank's constant, g is the gyromagnetic ratio,  is the out-of-plane anisotropy field of the magnetic region to switch, and V is the volume of the free layer. For most applications, the spin polarized current must be as small as possible.
Thermal stability is a function of the perpendicular anisotropy field as shown in equation (2):
                    Δ        =                                            M              s                        ⁢                          VH                                                k                  eff                                ,                ⊥                                                          2            ⁢                                                  ⁢                          k              B                        ⁢            T                                              (        2        )            where kB is the Boltzmann constant and T is the temperature. In both of the in-plane and out-of-plane configurations represented in FIG. 1, the perpendicular anisotropy field of the magnetic element is expressed in equation (3) as:
                              H                                    k              eff                        ,            ⊥                          =                              -                          DM              s                                +                                    2              ⁢                                                          ⁢                              K                U                                  ⊥                                      ,                    s                                                                                                      M                s                            ⁢              d                                +                      H                          k              ,              χ                                                          (        3        )            where D (approximately 4π) is the demagnetizing factor of the structure, Ms is the saturation magnetization, d is the thickness of the magnetic element, Hk,χ is the crystalline anisotropy field, and  is the surface perpendicular anisotropy of the top and bottom surfaces of the magnetic element. In polycrystalline materials where grains are randomly oriented, Hk,χ is the sum of the crystalline anisotropy of all the grains defining the region of interest. When the grains are large, Hk,χ can be significant whereas when the grains are small or the material is amorphous, this crystalline contribution to the total anisotropy field is small. From equation (3), one can see that the crystalline anisotropy of the material plays a detrimental role when the origin of the perpendicular anisotropy is mostly interfacial.
In the absence of strong crystalline anisotropy, the perpendicular anisotropy field of a magnetic layer is dominated by the shape anisotropy field on which little control is available. At a given thickness, lower magnetization saturation decreases shape anisotropy and the spin-polarized switching current but also decreases thermal stability which is not desirable. Therefore, an improved configuration for a magnetic element is needed that provides improved thermal stability for a free layer with perpendicular magnetic anisotropy. In other words, it is desirable to increase the perpendicular anisotropy field in a perpendicular-to-plane structure if one wants to increase thermal stability independently of moment or volume of the magnetic layer, and without affecting the critical current. There is presently no teaching as to how perpendicular magnetic anisotropy can be enhanced at first and second free layer interfaces with adjoining layers in a MTJ stack while selectively crystallizing portions of the free layer adjacent to these interfaces, and maintaining amorphous character (lower magnetic moment) in a middle portion of the free layer.